Abstract

A series of two- and three-dimensional idealized numerical experiments are conducted to examine the effects of different physical processes upon the development of the thermally driven regional-scale circulations over mountainous terrain simulated in Part I. The goal of this paper is to understand the conditions that enhance or suppress the formation of a westward-propagating density current within the mountain boundary layer. This current evolves from the Front Range mountain-plain circulation and was found in Part I to be responsible for unusual wind phenomena observed at mountaintop locations during the Rocky Mountain Peaks Experiment over western Colorado.

The idealized experiments show that the westward-propagating density current is a robust feature under summertime conditions of weak ambient flow and is initiated by differential heating across the Colorado Front Range between the plains and the intermountain region. In addition, the longevity of the thermally driven circulation system induces a steady southerly flow component, which persists over the intermountain region at night after the density current propagates away. The unique topography of the Colorado Rocky Mountain barrier—which features low plains on the east, a high dividing range, and a high plateau on the west—enhances the development of the current. The westward-propagating disturbance also develops over a range of low-level ambient wind speed, direction, and shear but is suppressed with low-level westerly flow, which also weakens the development of its progenitor, the Front Range mountain-plains solenoid.

Low-level stratification affects the depth and strength of the Front Range mountain-plains solenoid, which is most energetic in summertime conditions of near-neutral stability below 50 kPa. High stability in the lower troposphere suppresses the vertical development of the solenoid but increases the baroclinicity across the Front Range generated by surface heating, thereby still producing a significant density-current disturbance. Wet soil over the high terrain west of the Front Range also suppresses the formation and strength of the Front Range solenoid, while wet soil along the eastern slope of the Front Range and eastern plains with drier conditions over the high mountain terrain greatly enhances the baroclinicity within the solenoid and the subsequent density-current evolution. This couplet acts as an efficient conveyer of low-level moisture into the mountain region.

Abstract

Previous studies have shown liquid water potential temperature to be an inappropriate choice for a thermodynamic variable in a deep cumulus convection model. In this study, an alternate form of this variable called ice-liquid water potential temperature (θ_u ) is derived. Errors resulting from approximations made are discussed, and an empirical form of the θ_u equation is introduced which eliminates much of this error. Potential temperature lapse rates determined in saturated updrafts and unsaturated downdrafts by various θ_u approximations, an equivalent potential temperature approximation and a conventional irreversible moist thermodynamic approximation are then compared to the potential temperature lapse rate determined from a rigorously derived reversible thermodynamic energy equation. These approximations are then extended to a precipitating system where comparisons are again made. It is found that the errors using the empirical form of the θ_u equation are comparable to those made using conventional irreversible moist thermodynamic approximations. The advantages of using θ_u as an alternative to θ in deep convection and second-order closure models also are discussed.

Abstract

Observations collected during the Oklahoma–Kansas PRE-STORM experiment are used to document the evolution and structure of a mesoscale vortex couplet that developed in the mesoscale convective system that occurred on 16–17 June 1985. The evolution of the circulations was captured by dual-Doppler radar observations for 1.4 hours. This allowed an evaluation of the various terms of the vertical vorticity equation, which give insight into the mechanisms that are important in the generation of the circulations. The primary mechanism responsible for the formation of the observed vortices was the interaction of the larger-scale flow with low level momentum transported to higher levels by multiple convective updrafts. As a consequence vertical shear of the horizontal wind was important to initial vorticity production. The vorticity generated in this manner was subsequently increased in strength due to middle level convergence. When the convection weakened and dissipated, the primary source of vorticity was removed, and because this was an unbalanced circulation on a scale less than the Rossby radius of deformation, the vortex broke up and spun down. Comparisons are made with other documented cases, and differences and similarities are pointed out. It is hypothesized that this circulation is a common kind in precipitating mesoscale systems, which has hitherto largely been undetected because its size is too large to be easily observed in a Doppler radar network set up to study thunderstorms, yet too small to be detected by standard sounding networks or most research sounding networks.

Abstract

The 19 July 1993 mesoscale convective system (MCS), discussed in Part I, was simulated using the Regional Atmospheric Modeling System (RAMS). The model was initialized with variable physiographic and atmospheric data with the goal of reproducing the convective system and its four-dimensional environment. Four telescopically nested, moving grids allowed for horizontal grid spacings down to 1.67 km on the cloud resolving grid. Comparisons with the analysis show that the propagation, evolution, and structure of this MCS were well simulated.

The simulation is used to further investigate the interactions between this MCS and its surrounding environment. In Part I, the Doppler-derived winds indicated that upshear (westward) propagating gravity waves left upper-tropospheric front-to-rear and midtropospheric rear-to-front flow perturbations in their wake. A similar flow structure developed in the simulated MCS, and unlike the Doppler results, the low-frequency waves that produced it were resolved in the data. In the simulation, much of the convectively generated temperature and momentum perturbations propagated westward with the waves, leaving a warm wake in the clear air trailing the system. Although the gravity waves traveled rearward, the perturbation flow in their wake was not strong enough to reverse the upper-tropospheric storm-relative winds. Thus, most of the anvil condensate advected ahead of the convective line.

As the MCS encountered the low-level jet, the midtropospheric upward mass flux increased, but gravity wave motions became less detectable. The upper-tropospheric anvil pushed westward into the strong flow as the system expanded into a characteristically oval shape. Temperature and momentum perturbations propagated rearward along with the anvil in a disturbance that resembled an advective outflow. Unlike the gravity waves, this disturbance became almost stationary with respect to the ground, and it retained its continuity through the rest of the simulation. Vertical cross sections indicate that a large slab of convectively processed air had detrained into the upper troposphere. Prior to this event, much of the warm temperature anomalies generated within the convective towers either remained in the updrafts, or propagated outward with the gravity waves. Early on, individual updrafts were relatively erect and although condensate did detrain eastward in the forward anvil, the temperature anomalies did not propagate with it. In contrast, convective updrafts associated with the expanding oval anvil disturbance were more continuous, and they tilted strongly westward with height.

Abstract

A prolonged orographic precipitation event occurred over the Sierra Nevada in central California on 12–13 February 1986. This well-documented case was investigated via the nonhydrostatic version of the Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). The two-dimensional, cross-barrier simulations produced flow fields and microphysical structure, which compared well with observations. The feasibility of producing quantitative precipitation forecasts (QPF) with an explicit cloud model was also demonstrated.

The experiments exhibited a profound sensitivity to the input sounding. Initializing with a sounding, which is representative of the upstream environment, was the most critical factor to the success of the simulation. The QPF was also quite sensitive to input graupel density. Decreasing the density of graupel led to increases in the overall precipitation. Sensitivities to other microphysical parameters as well as orography and dynamics were also examined.

Abstract

Theoretical analysis shows that when water activity is larger than its threshold value and the dry radius of a particle is larger than 0.005 µm, the deviation of curvature correction from unity can be accurately represented by the product of two terms, with one term strongly depending upon water activity and the other depending upon dry radius. Moreover, experimental data show that the water-activity-dependent term can be approximated by linear and one-third power functions of water activity. According to the approximation made to curvature correction, water activity is solved as analytical functions of relative humidity (RH). The analytically solved water activity is then used to compute particle equilibrium sizes using a known (observed) relationship between water activity and water uptake by unit mass of dry material. The accuracy of equilibrium sizes calculated with this method is checked with seven typical classes of aerosols. Results show that when RH ≤ 99.99%, the equilibrium radius computed with this method is accurate to within 3% (6%) if the dry radius of a particle is larger (smaller) than 0.02 µm and that when RH > 99.99%, equilibrium sizes can be estimated with an accuracy higher than 84%. Analytical approximation formulas are also derived for calculating critical equilibrium radii and critical supersaturation. The maximum relative errors of these formulas range from 3% to 15%.

Abstract

This study employs a revised version of the Colorado State University three-dimensional numerical cloud scale model to investigate the observed behavior of deep convection over South Florida on 17 July 1973. A brief description of recent model improvements is made. A combined balance and dynamics initialization procedure designed to introduce variable magnitudes and distributions of low-level wind convergence to the initial fields is described.

Using radiosonde and PIBAL data collected by the NOAA/ERL Florida Area Cumulus Experiment (FACE) and the National Weather Service at Miami on 17 July 1973, composite wind, temperature, pressure and moisture profiles were constructed to depict the state of the atmosphere at the time of deep convection. Mesoscale convergence was estimated from results of a mesoscale model simulation of low-level sea breeze convergence made by Pielke (personal communication) for the same case study day. Several numerical simulations were performed using the sounding data as a basic state. The initial magnitude and distribution of low-level convergence was varied and the sensitivity of the model to some micro-physical parameters was examined.

The results of the numerical experiments show that (i) the magnitude of surface convergence over a finite area has a pronounced influence on the simulated storm circulation, the eddy kinetic energy of the storm and the total rainfall of the storm system; (ii) the horizontal distribution of convergence has a relatively large effect on the rates of entrainment into the updraft below 5 km MSL resulting in significant modulations in predicted precipitation, but only moderate changes in storm kinetic energy; (iii) variations in terminal velocity of precipitation associated with the introduction of the ice phase has only a minor effect on precipitation and total kinetic energy of the storm; and (iv) increased rain evaporation rates result in a moderate increase in the kinetic energy of the simulated storm, but at the expense of surface precipitation. Pressure forces are also shown to play an important role in initiating downdrafts and in biasing the direction of downdraft-associated outflow. Implications of these results to the modification of convective clouds are discussed.

Abstract

Using the one-dimensional cumulus model developed by Cotton, predictions of the effects of seeding cumulus clouds were performed during the month of July 1973 as part of the Experimental Meteorology Laboratory's Florida Area Cumulus Experiment 1973 experiment. In Part I we compared seedability predictions with the Miami 1200 GMT soundings and soundings taken over the center of the experimental area (Central Site) at 1400 GMT. It was found that substantial differences between the two predictions occurred on a number of days in spite of the fact that the soundings are separated in time by only 2 h and in space by only 110 km.

In this paper we compare seedability predictions with the MIA 1200 GMT soundings and the CS 1800 GMT soundings. The CS 1800 GMT soundings were assumed to be representative of conditions over the experimental area during the period of operation of the experiment. We found that the predictions with the MIA 1200 GMT soundings were, on the average, more representative of conditions over the center of the experimental area during the period of operation of the experiment than were the predictions with the CS 1400 GMT soundings. The results of this study indicate that the choice of a sounding site and sounding time to be used for prediction of seeding effects over an experimental area must be carefully considered in the design of the experiment.

Abstract

Using the one-dimensional cumulus model developed by Cotton, predictions of the effects of seeding cumulus clouds were performed during the month of July 1973 as a part of the Experimental Meteorology Laboratory FACE 1973 Experiment. The calculations were performed with the Miami 1200 GMT soundings and soundings taken in the interior of Florida at 1400 GMT at the so-called Central Site (CS) location.A comparison of “seedability” predictions using the Miami 1200 GMT and CS 1400 GMT soundings have shown that substantial differences between the two seedability predictions occur on a number of days in spite of the fact that the soundings are separated in time by only 2 h and in space by only 110 km. The differences can be attributed to the frequent intrusion of dry air masses of varying height and thickness. The intensity of the dry layers is generally greatest over the higher-latitude CS location. The greatest differences between the two soundings, and hence the greatest difference between the predicted seeding effects, occurs during periods of transition from a disturbed, westerly flow regime to a well-defined, deep, easterly flow regime.

Abstract

Nine clouds are simulated by perturbing Florida Area Cumulus Experiment (FACE) field soundings employing the Colorado State University cloud model. After a cloud similar in size to the one observed is initiated, glaciation is simulated in experiments designed to study the mechanisms by which glaciation is communicated to the subcloud boundary layer. Numerical model results show that the vertical pressure mechanism consisting of hydrostatic and dynamic pressure gradient force and “pressure buoyancy” is present, as is the downdraft mechanism, but they are secondary to loading, temperature buoyancy, water vapor buoyancy and the horizontal dynamic forces on the scale of a single deep convective cloud. The communication mechanism that has the most sustained and coherent influence upon the subcloud layer is the settling and evaporation of precipitation. A clear implication of this study to weather modification is that for dynamic seeding to have a significant influence upon the upscale growth of a cloud system, artificially triggered explosive growth of relatively weak convective towers must also be aimed at a carefully designed increase in the rainfall from those clouds.